The present invention relates to circuitry for charging nickel metal hydride (NiMH) and nickel cadmium (Ni--Cd) batteries.
Nickel metal hydride batteries are now a fairly mature battery technology, and continue to provide competitive amounts of power storage per unit volume. (Lithium-ion batteries provide superior power storage for unit weight, but are substantially more expensive than nickel metal hydride batteries.) See generally Linden, HANDBOOK OF BATTERIES (2.ed. 1995), which is hereby incorporated by reference.
Traditionally, NiCd or NiMH batteries are charged at a constant current or a constant power approximately equivalent to their "C rate". (The "C rate" is the current which would theoretically fill the battery's capacity in one hour. Thus, for a 1.7 Ampere-hour battery the C rate is 1.7 Amperes.)
When a discharged battery is recharged, it initially converts the active material in the electrodes to a higher valence state and thus stores energy chemically. When all of the active material is converted and the battery voltage increases, a secondary oxygen generating chemical reaction occurs. This oxygen which is generated from the positive electrode migrates to the negative electrode where it reacts to form water and heat. This is called an overcharge reaction.
The rate at which oxygen is generated is proportional to the current passing through the battery, and the rate at which the oxygen is recombined at the negative electrode is a function of cell design. It is important to throttle back the current when the battery reaches the overcharge (oxygen generating) condition, since otherwise excessive gas pressure within the cells can cause them to activate their safety vents (.about.100 psi). This results in loss of electrolyte and premature failure of the cell.
The traditional methods of terminating fast charge currents and throttling back to the acceptable C/10 or less maintenance currents were:
1.) By sensing the temperature rise of the battery and reducing charge current when the battery reached 40-45.degree. C., or PA1 2.) By sensing the battery voltage reduction from its peak voltage (-dV) equivalent to about 10-20 mV per cell for a given battery pack. This is ultimately related to the rise of battery pack temperature, since it is the self heating of the battery pack which causes the battery pack voltage to fall. PA1 Minimal gas generation which results in minimum internal pressure and minimum heat generation. This will reduce the likelihood of a cell venting, and the lower operating temperature will prolong the service life of the battery. PA1 Reduced exposure of the cell internals to oxidizing conditions. Thus this method of fast charge termination should be particularly advantageous for NiMH c ells whose components are very sensitive to oxidation at elevated temperatures. PA1 Cells can be charged at a fast rate (approximately 1C), during only the portion of the charge cycle when they can accept it. PA1 Temperature rise during charging is minimized In particular, temperature rise under over-charging conditions is minimized. PA1 Cells can be fully charged within 60-90 minutes without adverse temperature rise. PA1 The charging process is self regulating (regardless of power available), though may have to monitor ambient temperature. PA1 This invention provides a method of fast charging (in about one hour) a sealed nickel cadmium (NiCd) or a sealed nickel metal hydride (NiMH) battery in a way which minimizes the heat generated by the battery and which is less abusive to the battery and should result in a longer battery service life.
Thus both of these methods rely on heating within the battery. However, heating within the battery is inherently undesirable. By the time the cell surface temperature reaches 45.degree. C., a significant build up of heat within the battery is causing degradation of internal components, such as the separator. Also, the cell temperature lags behind the internal pressure increase and could result in the cell venting before the cutoff temperature is reached. This is likely when charging a battery at the low end of its temperature specification.
Moreover, the oxygen generated during the overcharge reactions cause an oxidizing atmosphere in the interior of the cells. This too can degrade the internal components of some battery cells.
Prior art FIG. 1 shows typical curves for voltage, temperature, and current during conventional charging of a NiMH battery. After the voltage has initially risen to a stable value V.sub.1 (in the neighborhood of 1.2 volts per cell, in this example) at time t.sub.1, the voltage stays fairly constant (although unregulated), until it begins to rise again, towards the end of the charging cycle, at time t.sub.2. At about this time (if the charging is being performed under constant-power conditions) the current through the battery begins to decrease at a faster rate than previously. At a time shown as t.sub.3, the voltage reaches its maximum V.sub.max (around 1.5 volts per cell, in this example), and thereafter declines. Either the decline in voltage after time t.sub.3, or the rise in voltage after time t.sub.2, is typically used to detect the end of charge. However, as the third curve in FIG. 1 shows, the temperature increases dramatically from time t.sub.2 on. Thus, this conventional way of detecting completion of nickel metal hydride charging imposes a substantial thermal load on the system, and may also be difficult to implement optimally where a battery is disconnected and reconnected to the charging circuit frequently.
FIG. 1 shows charging under constant power conditions, but similar voltage and temperature behavior would appear under constant current conditions.
Fast Charging Method
The present application teaches a new method for charging nickel metal hydride and comparable batteries. As shown in FIG. 2, the applied voltage is clamped to a value which is regulated, in a temperature-dependent way, to always be less than the gassing voltage for the particular conditions being experienced by the cell. Thus the endpoint must be detected in some other way, or the battery can simply be allowed to stabilize at this clamped voltage. Since gassing does not occur, nor the massive rise in temperature used in previous methods, detection of the end of charging stage is not critical.
Thus the internal pressure and temperature increase is minimized by reducing the generation of oxygen in the cells during overcharge. When the battery voltage increases to the clamped value, the current falls off to a low value that produces very little oxygen and therefore very little heat. The oxygen that is produced is recombined quickly and therefore causes little pressure build up within the cell. Since the battery voltage is inversely proportional to temperature, this clamping voltage is temperature compensated. Tests have confirmed that this method of clamping the charge voltage results in a minimal rise in battery temperature while inputting a full charge.
Some of the advantages of this method are: